If the intake of metabolic fuels is equivalent to energy expenditure, there is a state of energy balance. Overall, there will be equal periods of fed-state metabolism (during which nutrient reserves are accumulated as liver and muscle glycogen, adipose tissue triacylglycerols and labile protein stores) and fasting-state metabolism, during which these reserves are utilized. Averaged out, certainly over several days, there will be no change in body weight or body composition.
By contrast, if the intake of metabolic fuels is greater than is required to meet energy expenditure, the body will spend more time in the fed state than the fasting state; there will be more accumulation of nutrient reserves than utilization. The result of this is an increase in body size, and especially an increase in adipose tissue stores. If continued for long enough, this will result in overweight or obesity, with potentially serious health consequences — see Chapter 6.
The opposite state of affairs is when the intake of metabolic fuels is lower than is required to meet energy expenditure. Now the body has to mobilize its nutrient reserves, and overall spends more time in the fasting state than in the fed state. The result of this is undernutrition, starvation and eventually death — see Chapter 8.
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